1. Field of the Invention
The present invention relates to a magnet roller for use in the developing unit of an electrophotographic device or electrostatic recording device using magnetic toner.
2. Description of the Prior Art
Heretofore, in the field of electrophotographic devices and electrostatic recording devices such as copiers, printers and facsimiles, there has been known a developing method which comprises forming an electrostatic latent image on a latent image holder such as a photosensitive drum and visualizing the electrostatic latent image by supplying magnetic toner to the electrostatic latent image by means of a developing device.
FIG. 16 is a diagram showing the constitution of a conventional developing device 40. The developing device 40 comprises a developing roller 50 which contains a cylindrical magnet roller 52 magnetized to a predetermined magnetizing pattern in a rotating cylindrical sleeve 51. The developing device 40 sucks up magnetic toner T stored in a developing container 53 to the surface of the sleeve 51 based on the magnetizing pattern and carries it, forms a uniform thin layer of the magnetic toner T by means of a layer forming blade 54, and supplies the magnetic toner T on an electrostatic latent image on the surface of a photosensitive drum 60 to visualize it by a so-called jumping phenomenon that the magnetic toner 5 is jumped on the photosensitive drum 60 by the magnetic force characteristics of the magnet roller 52. The magnetic toner T remaining on the surface of the sleeve 51 is separated from the sleeve 51 and collected thereafter.
In the magnet roller 52, the above magnetizing pattern is designed such that a sucking pole 52a for sucking up the magnetic toner T and a collection pole 52b for separating the magnetic toner T are made the same in polarity to carry out the collection and sucking of the magnetic toner T by repulsive force between the two poles. A zero-gauss zone where the peak magnetic force becomes 100 gauss or less is formed between the above two poles to separate them from each other to improve the recovery of the remaining magnetic toner T.
The above magnet roller 52 is produced by injection molding or extrusion molding into a roll form a pellet-shaped bond magnetic composition prepared by mixing magnetic powders such as ferrite with a thermoplastic resin such as nylon or polypropylene using a metal mold having a magnetic field formed around a cavity and magnetizing the surface of the roll to a desired magnetization pattern.
Various magnetization patterns are required of the magnet roller according to the specifications of a copier. Therefore, besides the above-described magnet roller produced by magnetizing a roller integrally molded out of a bond magnet composition to a desired magnetization pattern, a cylindrical magnet roller produced by joining together a plurality of bar-shaped magnet pieces whose outer magnetic poles have been made N or S poles by magnetization and whose section is fan-shaped in such a manner that sides of the magnet pieces are contacted to one another.
Along with recent progress in electrophotographic devices and the like, a more complicated magnetization pattern has been desired of a magnet roller. The magnet roller of the prior art has a limit in the design of a magnetization pattern and cannot meet such demand in some cases. Particularly in a magnet roller having a magnetization pattern in which a zone where the peak magnetic force becomes 100 gauss or less (to be referred to as xe2x80x9czero-gauss zonexe2x80x9d hereinafter) is formed at almost an intermediate position between the adjacent poles of the magnetization pattern, the pole positions after magnetization are shifted from predetermined pole positions (designed pole positions) by production differences including a difference in the orientation of magnetic powders at the time of molding, whereby the dislocation of the zero-gauss zone of the magnetization pattern occurs and it is difficult to obtain a magnet roller having a desired magnetization pattern. Therefore, when development is carried out with a developing roller comprising a magnet roller having a magnetization pattern with a dislocated zero-gauss zone, a clear image cannot be obtained.
This problem is also seen in the above-described magnet roller constructed by joining together a plurality of magnet pieces. Since the zero-gauss zone is dislocated by differences in magnetization state among the magnet pieces, a desired magnetization pattern cannot be obtained.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a magnet roller having a magnetization pattern which rarely experiences the dislocation of a zero-gauss zone though it is simple in structure.
According to a first aspect of the present invention, there is provided a magnet roller which comprises a magnet body integrally molded out of a resin magnet composition prepared by mixing and dispersing magnetic powders in a resin binder and a shaft portion projecting from both ends in an axial direction of the magnet body and which has a magnetization pattern with a zero-gauss zone between a pair of magnetic poles having the same polarity which are adjacent to each other in a circumferential direction (for example N and N poles in a magnet roller having N-S-N-N-S poles), wherein a cut-out portion extending in an axial direction is formed on the side of the magnet body at the predetermined position of the zero-gauss zone. Thereby, the position of the zero-gauss zone can be stabilized by suppressing a change in magnetic force between the above magnet poles having the same polarity.
According to a second aspect of the present invention, there is provided a magnet roller, wherein the size of the cross section of the cut-out portion is 5% or more of the cross-sectional area of the magnet body.
According to a third aspect of the present invention, there is provided a magnet roller, wherein an area having a difference of 40 gauss or less from the minimum magnetic force point of the magnetization pattern in the zero-gauss zone is 30xc2x0 or more in terms of the angle of the roller.
According to a fourth aspect of the present invention, there is provided a magnet roller, wherein the number of magnetic poles of the magnetization pattern is an odd number.
According to a fifth aspect of the present invention, there is provided a magnet roller which comprises a magnet body constructed by arranging in a circumferential direction a plurality of magnet pieces molded out of a resin magnet composition prepared by mixing and dispersing magnetic powders in a resin binder and shaft portions projecting from both ends in an axial direction of the magnet body and which has a magnetization pattern with a zero-gauss zone between a pair of magnet pieces adjacent to each other in a circumferential direction, wherein the magnet pieces are arranged in such a manner that the joint between the adjacent magnet pieces of two magnet poles is located at the predetermined position of the zero-gauss zone to stabilize the position of the zero-gauss zone.
According to a sixth aspect of the present invention, there is provided a magnet roller, wherein the difference of angle between the position of the zero-gauss zone and the position of the joint is 30xc2x0 or less.
According to a seventh aspect of the present invention, there is provided a magnet roller which comprises a magnet body constructed by arranging in a circumferential direction a plurality of magnet pieces molded out of a resin magnet composition prepared by mixing and dispersing magnet powders in a resin binder and shaft portions projecting from both ends in an axial direction of the magnet body and which has a magnetization pattern with a zero-gauss zone between a pair of magnet pieces adjacent to each other in a circumferential direction, wherein a space is formed between the above adjacent magnet pieces of two magnetic poles and located at the predetermined position of the zero-gauss zone to stabilize the position of the zero-gauss zone.
According to an eighth aspect. of the present invention, there is provided a magnet roller, wherein rare earth alloy powders are used as the magnetic powders to obtain high magnetic force even when the roller has a small diameter.
According to a ninth aspect of the present invention, there is provided a magnet roller, wherein the rare earth alloy powders are anisotropic rare earth alloy powders.
According to a tenth aspect of the present invention, there is provided a magnet roller, wherein the anisotropic rare earth alloy powders are any one of anisotropic Smxe2x80x94Fexe2x80x94N alloy powders and anisotropic Ndxe2x80x94Fexe2x80x94B alloy powders or a mixture thereof.
The above and other objects, features and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.